1. Field of the Invention
The present invention relates to a decoding apparatus, a decoding method, and a decoding program for decoding a low-frequency component from a first coded data obtained by coding a low-frequency component in an audio signal, and decoding the high-frequency component of the audio signal from a second coded data that is used to decode a high-frequency component in the audio signal and the low-frequency component.
2. Description of the Related Art
In recent years, in order to code audio or music, High-Efficiency Advanced Audio Coding (HE-AAC) has been used. The HE-AAC format is an audio compression format mainly used in Moving Picture Experts Group phase 2 (MPEG-2), or Moving Picture Experts Group phase 4 (MPEG-4).
In the HE-AAC, a low-frequency component in a frequency of an audio signal (signal relating to audio, music, etc.) to be coded is coded according to Advanced Audio Coding (AAC), and a high-frequency component in the frequency is coded according to Spectral Band Replication (SBR). In the SBR format, the high-frequency component in the frequency of the audio signal can be coded using smaller number of bits than that used in the other formats by coding only a part that is hard to predict from the low-frequency component in the frequency of the audio signal. Hereinafter, the data coded according to the AAC format is referred to as AAC data, and the data coded according to the SBR format is referred to as SBR data.
Now, an example of a decoder that decodes data (hereinafter, referred to as HE-AAC data) coded according to the HE-AAC format is described. FIG. 19 is a functional block diagram illustrating a configuration of a known decoder. As illustrated in FIG. 19, a decoder 10 includes a data separation section 11, an AAC decoding section 12, an analysis filter section 13, a high-frequency generation section 14, and synthesis filter section 15.
The data separation section 11 is a processing section that when HE-AAC data is acquired, separates AAC data and SBR data contained in the acquired HE-AAC data respectively, outputs the ACC data to the AAC decoding section 12, and outputs the SBR data to the high-frequency generation section 14.
The AAC decoding section 12 is a processing section that decodes AAC data and outputs the decoded AAC data as AAC output audio data to the analysis filter section 13. The analysis filter section 13 is a processing section that calculates a characteristic between time necessary for the low-frequency component in the audio signal and a frequency based on the ACC audio data acquired from the AAC decoding section 12, and outputs the calculation result to the synthesis filter section 15 and the high-frequency generation section 14. Hereinafter, the calculation result outputted from the analysis filter section 13 is referred to as low-frequency component data.
The high-frequency generation section 14 is a processing section that generates a high-frequency component in the audio signal based on the SBR data acquired from the data separation section 11 and the low-frequency component data acquired from the analysis filter section 13. Further, the high-frequency generation section 14 outputs the data of the generated high-frequency component as high-frequency component data to the synthesis filter section 15.
The synthesis filter section 15 is a processing section that synthesizes the low-frequency component data acquired from the analysis filter section 13 with the high-frequency component data acquired from the high-frequency generation section 14 and outputs the synthesized data as HE-AAC output audio data.
FIG. 20 is a view for outlining a processing performed in the decoder 10. As illustrated in FIG. 20, the decoder 10 replicates a part of low-frequency component data, and adjusts an electric power of the replicated data to generate high-frequency component data. Then, the decoder 10 synthesizes the low-frequency component data with the high-frequency component data to generate HE-AAC output audio data. As described above, the HE-AAC data (audio signal, etc.) that is coded according to the HE-AAC format is decoded as the HE-AAC output audio data by the decoder 10.
In Japanese Laid-open Patent Publication No. 2005-338637, a technique for improving auditory quality is disclosed. In the technique, a value of a scale factor in an audio signal is adjusted to correct a mismatch between powers of the audio signal before coding and after coding.
However, the above-described known technique cannot solve a problem that after an audio signal that contains an attack sound (signal that has a sharp amplitude change) is coded, when the coded audio signal is decoded, it is not possible to appropriately decode a high-frequency component in a frequency of the audio signal.
The problem in the known technique is specifically described. FIGS. 21A and 21B are views for explaining the problem in the known technique. As illustrated in FIGS. 21A and 21B, in a case where an audio signal that contains an attack sound whose amplitude sharply changes in an extremely short duration is coded according to the SBR format, because of characteristics in the SBR format, a time domain where the attack sound is generated can be extremely short (or, a temporal resolution in the SBR format becomes poorer than that in the AAC format) as compared to a time domain divided according to the SBR format. Then, the power in the time domain that contains the attack signal is averaged, and the attack sound is coded in a state the attack sound is temporally extended.
That is, it is very important problem to be solved to correct the high-frequency component in the coded audio signal and appropriately decode the audio signal even if the high-frequency component in the audio signal containing the attack signal is not appropriately coded according to the HE-AAC format. Especially, it is important to accurately correct the duration of the attack sound contained in the high-frequency components even if a steady component other than the attack sound exists in the low-frequency components that are coded according to the AAC format.